1. Field of the Invention
The present invention relates to gaskets as sealing devices, and particularly relates to a gasket for fuel cells, such as stacked fuel cells. The gasket supports a sheet, such as an electrolytic membrane, in a narrow region and seals working gasses and fluids, such as oxygen, nitrogen, hydrogen and water.
2. Description of the Related Art
In general, each "cell" of a fuel cell includes a pair of porous bipolar plates or collector separator electrodes and a pair of membrane and electrode assemblies (MEAs). Each MEA is composed of a polymer electrolyte membrane, a catalytic layer, and a reactive electrode layer. The MEA is sandwiched between two bipolar plates. The functional requirements in such a composite configuration or cell are, for example, a constant distance between the two adjacent bipolar plates, a high level of hermetic or low permeation sealing, precluding evaporation of water and drying of the polymer electrolyte membrane, and easy assembly and disassembly. Several adjacent "cells" form a fuel cell "stack".
Normally, the fuel cell stack is sealed with a curable bonding agent. This seal is effective initially, but it has a serious drawback because deteriorated curable bonding agent seals cannot be easily replaced with new ones.
Solutions for solving this problem are, for example, to provide sealing between fuel cell stacks using gaskets, such as disclosed in Japanese Patent Application Laid-Open Nos. 9-231987, 7-227220 and 7-153480, or using a composite gasket composed of a rubber sheet and a cellular or sponge layer, as disclosed in Japanese Patent Application Laid-Open No. 7-312223. These gaskets add to the overall thickness of the fuel cell stack and do not allow for easy assembly and disassembly.
Another solution includes using a composite gasket composed of a metal frame or sheet and a rubber layer. This has the following disadvantages:
A. Gaseous constituents in the fuel cell and cooling water react with the metal sheet to generate contaminating ions. This causes decreased electrical generating efficiency of the fuel cell.
B. Since the metal frame is relatively thick, in the range of 0.5 to 2 mm, a large fuel cell stack which includes approximately 100 cells is heavy and large.
C. Any distortion of the metal frame such as by warping will make positioning and assembly of the polymer electrolyte membrane difficult. This distortion can occur when a seating stress is applied to ensure sealing and as a result the metal frame will warp. The warped part causes curvature of the metal frame of the gasket, and thus making assembly of the fuel cell more difficult.
D. Since a large area is sealed, the fuel cell assembly requires a large compressive force. However, the compressive force of sealing changes significantly as the compressed height of the seal changes slightly, resulting in unstable sealing characteristics.
When assembling fuel cells, polymer electrolyte membranes are apt to be contaminated with dust or the like due to handling the sheets directly. Dust or the like influences the generating efficiency of the fuel cell. It is also difficult to position the sheet correctly in the predetermined position in the fuel cell because the membrane is thin and soft.